1. Field of the Invention
The present invention relates to a semiconductor switch circuit which has a self-holding function and is capable of performing a large-current switching operation by a small controlling current.
2. Description of the Prior Art
By using a semiconductor switch such as a thyristor or a transistor in place of a mechanical contact switch, the performance of the switch circuit may be improved remarkably in speed, life, noise and compactness. The conventional semiconductor switch circuits, however, pose a problem with the on-off control thereof. In a semiconductor switch circuit using transistors, the base thereof is required to continue to be driven with a large current while it is in the on state. Another semiconductor switch circuit using a thyristor, on the other hand, has the characteristics of easy on-control and also the self-holding function, but the off-control thereof is impossible except with a thyristor of a special type. One of the off-controllable thyristors is called a gate turn-off thyristor (GTO). However, even in the case of a GTO, a larger driving power is required for off-control as compared with that for on-control, thus making on-off control difficult.
Under these circumstances, the Japanese laid-open patent publication No. 41482/75 suggests a method as shown in FIGS. 1 and 2 wherein a composite switch is made up of a thyristor and a transistor such that the off-control ability is improved by the current amplification effect of the transistor. In the circuit shown in FIG. 1, however, the off-control ability is greatly affected by the construction of the semiconductor devices making up the switch circuit. This restriction leads to the disadvantage of difficulty in circuit integration.
In FIG. 1, the complementary transistors Tr.sub.1 and Tr.sub.2, which are equivalent to a thyristor T, and a transistor Tr.sub.3, driven by the N gate (GN) of this thyristor T, make up a switch circuit. When the current amplification factor (h.sub.FE) of the transistor Tr.sub.3 is large enough, the greatest part of the conduction current of the switch circuit flows between the collector and emitter of the transistor Tr.sub.3, so that the current flowing in the thyristor T which is to be turned off is reduced, thus improving the off-control ability thereof.
The circuit of FIG. 1 is such that the bases and emitters of the transistors Tr.sub.2 and Tr.sub.3 are connected directly with each other respectively. Therefore, the base currents in the transistors Tr.sub.2 and Tr.sub.3 do not always flow equally, resulting in the lack of evenness of current flow depending on the relative difference in the magnitude of the internal resistances of the transistors Tr.sub.2 and Tr.sub.3. This is called a current hogging phenomenon which poses a problem in respect of electrical characteristics. In other words, if the base internal resistance of the transistor Tr.sub.2 is smaller than the base internal resistance of the transistor Tr.sub.3, substantially no base current flows in the transistor Tr.sub.3. As a result, the collector current of the transistor Tr.sub.3 is almost zero, thus lowering the off-control ability extremely. When the internal resistance of the transistor Tr.sub.2 is larger than that of the transistor Tr.sub.3, on the other hand, the base current of the transistor Tr.sub.2 is reduced and the driving current of the thyristor T is increased, resulting in a lower sensitivity.
For this reason, it is necessary to achieve a construction of transistors Tr.sub.2 and Tr.sub.3 as shown in FIG. 2 in which the current hogging phenomenon is unlikely to occur. According to the structure of the device shown in FIG. 2, however, the off-control ability and the breakdown voltage are in a trade-off relation, thus making it impossible to maintain switch devices high in both the off-control ability and breakdown voltage. The device shown in FIG. 2 is so constructed that a P-type emitter layer 11 making up an anode A, an N-type base layer 12 making up an N gate (not shown), a P-type base layer 13 making up a gate G, and an N-type emitter layer 14 making up a cathode K are formed on a P-type substrate 10, thus constituting a thyristor T, with the P-type substrate 10 being connected electrically with the cathode K. In FIG. 1, the transistor Tr.sub.2 is made up of the P-type emitter layer 11, the N-type base layer 12 and the P-type base layer 13 as the emitter, base and collector regions respectively. The transistor Tr.sub.3, on the other hand, is made up of the P-type emitter layer 11, the N-type base layer 12 and the P-type substrate 10 as the emitter, base and collector regions respectively. In this construction, the base-emitter circuits of the transistors Tr.sub.2 and Tr.sub.3 share a common region, so that the base currents of the transistors Tr.sub.2 and Tr.sub.3 are equal to each other, thus obviating the above-mentioned problem of the current hogging phenomenon. In spite of this advantage, the structure of the device shown in FIG. 2 has such a shortcoming that if the current amplification factor h.sub.FE of the transistor Tr.sub.3 is increased in order to improve the off-control ability, the breakdown voltage of the device is reduced. In other words, in order to increase the value h.sub.FE of the transistor Tr.sub.3, it is necessary to reduce the thickness of the base layer thereof, i.e., the distance between P-type emitter layer 11 and the P-type substrate 10 in FIG. 2 (hereinafter called the N base thickness). The breakdown voltage of the device (forward off breakdown voltage), however, becomes lower as the N base thickness decreases. As seen from the foregoing explanation, the conventional circuit shown in FIG. 1 is greatly limited in the realization of integrated circuitry and is thus capable of being used only in a certain field of application such as one involving a low breakdown voltage.